The present invention relates generally to x-ray imaging systems. More particularly, the present invention relates to systems and methods of adjusting focal spot positioning relative to a target within an imaging tube.
Traditional x-ray imaging systems include an x-ray source and a detector array. X-rays are generated by the x-ray source, pass through an object, and are detected by the detector array. Electrical signals generated by the detector array are conditioned to reconstruct an x-ray image of the object.
Computed tomography (CT) imaging systems include a gantry that rotates at various speeds in order to create a 360° image. The gantry contains a CT tube assembly that generates x-rays across a vacuum gap between a cathode and an anode. In order to generate the x-rays, a large voltage potential of approximately 150 kV is created across the vacuum gap allowing electrons, in the form of an electron beam, to be emitted from the cathode to the target portion of the anode. In the releasing of the electrons, a filament contained within the cathode is heated to incandescence by passing an electric current therein. The electrons are accelerated by the high voltage potential and impinge on the target at a focal spot, whereby they are abruptly slowed down, directed at an impingement angle α of approximately 90°, to emit x-rays through a CT tube window.
The cathode or electron source is typically a coiled tungsten wire that is heated to temperatures approaching 2600° C. The electrons are accelerated by an electric field imposed between the cathode and the anode. The anode, in a high power x-ray tube designed for current CT devices, is a tungsten target having a target face, that rotates at angular velocities of approximately 120 Hz or greater.
The focal spot has an associated location on a surface of the anode. The location of the focal spot, with respect to the gantry and CT detector assembly, is dependent upon the position of the target face with respect to an insert frame of the imaging tube, which is fixed to an outer frame or casing of the tube. The temperature of different elements of the anode, such as an anode rotor, stem, bearing, stud, hub, and thermal barrier, determine z-direction position of the target face, along an axis of rotation of the anode.
The focal spot location is controllably translated within the x-ray imaging tube in order to perform a double sampling technique. The double sampling technique is utilized to prevent aliasing effects in image reconstruction. It is desirable to prevent aliasing in order to generate quality images with minimum artifacts in x-ray imaging.
Double sampling refers to a sampling frequency of at least 2/a, where “a” is a third generation computed tomography (CT) scanner sampling distance of a scanned field. Sample frequency for the CT scanner is equal to 1/a, which is half the preferred Nyquist theorem sampling frequency of at least 2/a. Double sampling can be achieved by numerically evaluating two images. A first image is acquired with the detector in a default position and a second image is acquired after moving the detector by a distance of a/2 normal to the incident x-rays while maintaining position of the x-ray source. Equivalently, the two images needed for double sampling can also be obtained by laterally moving the focal spot between two exposures a distance that causes the subsequent x-ray image to move a distance of a/2 on the detector.
Double sampling is accomplished in conventional imaging systems by adjusting focal spot positioning on the target or surface of the anode, electronically without mechanical motion, via use of deflection coils or plates within an x-ray tube. The deflection coils and plates deflect an electron beam either by creating a local magnetic or electrostatic field.
A method of performing double sampling of each beam is to wobble an x-ray source or imaging tube by an amount that shifts each beam by one-half the space between the beams. Wobbling is mechanically equivalent to taking a second set of projections with the detector shifted to some odd multiple of one-half pitch of the detector. The detector is allowed to naturally rotate to a one-half pitch position while the x-ray source is repositioned, along a circumferential path of rotation of the source, back to a position where a first projection set of data was collected. Wobbling is generally within a plane of rotation of the gantry and along a tangent to the gantry rotation.
Wobbling may be performed by acquiring a first set of data with a focal spot in a first position on a first 360° scan and acquiring a second set of data with the focal spot shifted to a second position on a second 360° scan. Preferably, however, to avoid motion problems between adjacent samples, the x-ray beam is rapidly shifted between positions and each projection.
Due to limited amounts of available space within an imaging tube utilization of the deflection coils and plates is not feasible. The close proximity and the high voltage potential between the cathode and the anode render the deflection coils and plates impracticable.
Externally generated magnetic fields have been suggested for focal spot position adjustment and wobbling, which would allow use of current cathode/anode designs. However, in order to generate the magnetic fields, external components are required, which considerably increases weight of the imaging tube. Increase in weight limits feasible rotating speeds of CT imaging systems due to increases in loads experienced by gantry components. The increased loads degrade CT imaging tube performance.
It would therefore be desirable to provide a focal spot position adjusting system that is applicable to CT imaging, that is electronic, does not significantly increase weight of or occupy increased space within an imaging tube, and does not require use of deflection coils or plates.
Thermally induced growth of anode elements with increase in temperature is referred to as z-thermal. Z-thermal is tracked by various methods. Z-thermal is typically determined by estimating the position of the target face by calibrating a measured focal spot position with respect to power or total heat deposited in the target. Cool-down times are recorded and estimates can be made on focal spot positions, during operation, even after extended periods of not using the CT system. A CT device back-projection algorithm introduces corrections for focal spot motion since final image artifacts depend upon differences between a real focal spot location and an estimated focal spot location.
Target face position estimating can be inaccurate. Actual focal spot positioning can drift over time due to temperature changes in various components, amount and type of use of the components, whether a component is new or aged, system operating power level, system operating time, and other focal spot position affecting factors known in the art.
Another disadvantage with existing focal spot estimation is different CT x-ray tube designs require different focal spot motion calibration schemes, which must be developed, tested, and performed for each tube type and potentially for each design revision within a tube type. The calibration schemes are costly to implement, time consuming, and are potentially inaccurate since multiple anode behaviors occur with a specified anode temperature.
It is therefore also desirable to provide a system for accurately determining actual focal spot positioning.